Ian Poulter tried to join them but had a bad lie in a bunker at No. 16 and his approach rolled off the back edge of the green at the 18th. The two bogeys sank him from even par to 2 over after his round of 71.
Once everyone finished, Mickelson and Horschel remained the only players under par, at 1-under 139 at the halfway point of the championship.
''I'm three off the lead in the U.S. Open,'' Poulter said. ''And that's the difference of one hole. You can make birdie and someone can make double. I'm right in position and right there where I want to be. It's going to be a fun weekend.''
The cut line was 8 over, saving both defending champion Webb Simpson (5 over) and Masters champion Adam Scott (7 over). Third-round play was scheduled to start at 12:15 p.m., with threesomes teeing off at Nos. 1 and 11 in a tournament that fell behind schedule when storms moved through the Philadelphia area on Thursday.
Tiger Woods was tied for 13th at 3 over, still in the hunt as long as he can deal with the pain from a troublesome left elbow - as well as Merion's wicked rough and flummoxing greens that more than compensate for shorter-than-usual yardage for a U.S. Open.
''It's hard with the wind and the pin locations,'' Woods said after his second-round 70. ''They're really tough. ... We didn't think they were going to be as severe as they are.''
The top of the leaderboard was a study in contrast. Mickelson has won four majors. Horschel has won once on the PGA Tour, and that was less than two months ago.
Mickelson displayed his usual take-a-chance flair Friday. His round of 72 was the full package of par saves and makeable birdie putts that all went awry – until he finally sank one from 20 feet at the 18th, the hardest hole on the course, to tie him with Horschel seconds after the horn sounded to suspend play for the day.
''On 18, when you don't really expect to get one, I put the ball in a good spot and was able to roll one in,'' Mickelson said.
Horschel's path was much more straight-forward. He merely put the ball on the green in regulation 18 times out of 18, a stellar achievement for a regular tour event, much less a major championship. His 3-under 67 was the best round of the day.
''I wasn't in the zone, I was just focused on what I tried to do,'' said Horschel, who missed the cut in his only previous U.S. Open appearance, as a teenager in 2006. ''I didn't know I hit every green until I walked off 18. It's a cool thing.''
Yeah, pretty cool, considering he was one of only six players to shoot a red number in the second round at Merion. Justin Rose and Steve Stricker both carded a 69 to stand at even par, tied with Luke Donald (72) for second place.
Nearly half the field was still on the course when play was called due to darkness. Groups are allowed to complete the hole they're playing after the horn sounds, so Mickelson's birdie at 18 was the golf equivalent of hiking the football before time runs out and getting to complete the down. In fact, his group was so eager to finish the round that they negotiated with the group ahead for playing partner Keegan Bradley to hit his tee shot early at 18.
''We need to hit one tee shot so we could finish,'' Mickelson said. ''They moved out of the way, and Keegan hit a tee shot and they went back and finished the hole. ... It's nice when guys like that help out.''
Horschel's surge shouldn't be much of a surprise. He's having a breakout year, with six finishes in the top 10 on the PGA Tour, including his victory at the Zurich Classic in late April.
''I've acquired some patience, not as much as I wish I had,'' said the 26-year-old from Jacksonville Beach, Fla.
Still, he's no Mickelson. The most recent personal buzz about Lefty was his decision to attend his daughter's eighth-grade graduation ceremony on the eve of the Open, then fly cross-country overnight to play his first round Thursday on little sleep.
Horschel? Well, his official PGA Tour bio says that he ''read all four Twilight books in two weeks and is a believer in Bigfoot and UFOs.''
He's also said he's steadied his game with the help of sports psychologist Fran Pirozzolo, who helped convince Horschel to think of the U.S. Open as ''another tournament.''
''I know it's a big event. I know it's a historical event,'' Horschel said. ''But one thing that me and Fran have worked on is limiting the distractions.''